1. Field of the Invention
The present invention relates to a single-mode optical fiber suitable for a transmission line in digital communications and a method of fabricating the same.
2. Related Background Art
Conventionally, in optical communication systems adopting a single-mode optical fiber (referred to as xe2x80x9cSM optical fiberxe2x80x9d hereinafter) as their transmission line, light of 1.3-xcexcm wavelength band or 1.55-xcexcm wavelength band has often been used. Recently, from the viewpoint of lowering transmission loss, the use of 1.55-xcexcm wavelength band light has been increasing. Such an SM optical fiber applied to transmission lines for 1.55-xcexcm wavelength band light (referred to as xe2x80x9c1.55-xcexcm SM optical fiberxe2x80x9d hereinafter) has been designed such that its wavelength dispersion (phenomenon in which pulse wave broadens due to the fact that velocity of propagation of light varies according to wavelength) becomes zero (a dispersion-shifted fiber whose zero dispersion wavelength is set 1.55 xcexcm). Currently, as such a dispersion-shifted fiber, optical fibers having a refractive index profile of a dual-shape type such as that disclosed in Japanese Patent Publication No. Hei 3-18161 or a segmented-core type disclosed in xe2x80x9cRelation between Macrobending Losses and Cutoff Wavelength in Dispersion-Shifted Segmented-Core Fiber,xe2x80x9d Electronics Letter, No. 22, No. 11, p. 574, 1986 have been mainly used.
Also, in recent years, as long-distance transmission has become possible because of the advent of optical amplifier, in order to prevent the four-lightwave mixing, which is one of nonlinear optical effects, there has also been used an optical fiber in which the above-mentioned refractive index profile is modified so as to shift the zero dispersion wavelength from 1.55 xcexcm to the shorter or longer wavelength side. Here, the nonlinear optical effects refer to phenomenons in which signal light pulse is distorted in proportion to density or the like of optical intensity. They become a factor restricting the transmission rate.
It is an object of the present invention to provide a single-mode optical fiber having a configuration which realizes both reducing of dispersion slope and a sufficient mode-field diameter, and a method of fabricating the same. In order to attain such a single-mode optical fiber, the inventors have studied the conventional single-mode optical fiber as explained in the following.
Since the,conventional 1.55-xcexcm SM optical fiber has a zero dispersion wavelength set near 1.55 xcexcm, the wavelength dispersion thereof gradually increases as the signal light wavelength deviates farther from 1.55 xcexcm. In particular, in the case where the absolute value of its dispersion slope (e.g., differential coefficient, at the zero dispersion wavelength, of a curve indicating dispersion with respect to the signal light wavelength shown in each of FIGS. 13 and 14) is large, the ratio of increase in the absolute value of wavelength dispersion unfavorably becomes higher when the zero dispersion wavelength of the SM optical fiber or the wavelength of a light source deviates from 1.55 xcexcm. Also, in long-distance transmission, since a wavelength dispersion is intentionally generated in order to suppress the above-mentioned nonlinear optical effects, dispersion-shifted fibers in general are designed such that the zero dispersion wavelength thereof is slightly shifted from the signal light wavelength. Accordingly, in the SM optical fiber, which is a transmission line, it is necessary to lower the absolute value of dispersion slope or to effect dispersion compensation (in which signal light is made to pass through an SM optical fiber having a reverse dispersion characteristics) in the transmission line.
In wavelength-divided multiplex transmission (referred to as xe2x80x9cWDMxe2x80x9d hereinafter) in which a plurality of wavelengths are transmitted as being overlaid on each other in order to increase the transmission rate, since a plurality of wavelengths near the zero dispersion wavelength are used, when the dispersion slope has a large absolute value, the difference among wavelength dispersion values at respective wavelengths may increase so much that dispersion cannot be compensated for. Accordingly, it is important to lower the absolute value of dispersion slope.
In the refractive index profile of the conventional dual shape type or segmented-core type, as the absolute value of dispersion slope is made smaller, the mode-field diameter (referred to as xe2x80x9cMFDxe2x80x9d hereinafter) decreases or the bending loss increases, thereby making it inevitable for the lateral-pressure characteristic to deteriorate. As the connection characteristic deteriorates when the MFD is made too small, the lower limit thereof has conventionally been set. Also, in long-distance transmission using a light amplifier, nonlinear phenomenons are enhanced as MFD is lowered, thereby demanding increase in MFD. Also, deterioration in the side-pressure characteristic becomes an obstacle when SM optical fibers are formed into a cable. Accordingly, it is important to note that the reducing in absolute value of dispersion slope and the increase in MFD have a trade-off relationship therebetween.
In the refractive index profile of dual shape type or segmented-core type, the refractive index near the center of core region has a constant value or decreases toward the outside. The core of an SM optical fiber having such a refractive index profile comprises an inner core at the center portion thereof and an outer core having a lower refractive index than the inner core. In order to decrease the absolute value of dispersion slope in this refractive index profile, the following three kinds of methods have been known:
(1) Increase the outer core.
(2) Increase the ratio of inner core diameter/outer core diameter.
(3) Decrease relative refractive index difference of the inner core with respect to pure silica glass (where the outer core has a lower refractive index than that of the inner core).
Nevertheless, the method of (1) leads to decrease in MFD, whereas the methods of (2) and (3) cause bending loss (increase in transmission loss in the state where the optical fiber is bent) to increase.
The single-mode optical fiber (SM optical fiber) according to the present invention has a specific configuration in order to overcome the foregoing problems. Specifically, as shown in FIG. 1, an SM optical fiber 1 according to the present invention is an optical fiber mainly composed of silica glass, comprising a core region having a predetermined refractive index and a cladding portion 200 which is formed around the outer periphery of the core region and whose refractive index is set lower than that of the core region. The core region comprises a first core portion 110 whose mean relative refractive index difference with respect to the cladding portion 200 is a first value xcex94n1 and whose outer diameter is a; a second core portion 120 which is a glass region whose mean relative refractive index difference with respect to the cladding portion 200 is a second value xcex94n2 greater than the first value xcex94n1 and whose outer diameter is b, formed around the outer periphery of the first core portion 110; and a third core portion 130 which is a glass region whose mean relative refractive index difference with respect to the cladding portion 200 is a third value xcex94n3 which is smaller than the second value xcex94n2 and whose outer diameter is c, formed around the outer periphery of the second core portion 120.
In particular, in the SM optical fiber 1 according to the present invention, the first core portion 110 and the second core portion 120 satisfy the following relationship:
axc2x7(xcex94n2xe2x88x92xcex94n1)/(bxc2x7xcex94n2)xe2x89xa70.04xe2x80x83xe2x80x83(1)
therebetween.
Here, the refractive index profile 600 of the SM optical fiber 1 does not always have a clear step-like form. In such a case, the outer diameter a of the first core portion 110 is defined by a diameter in the boundary portion between the first core portion 110 and the second core portion 120 at which the relative refractive index difference is (xcex94n1+xcex94n2)/2; the outer diameter b of the second core portion 120 is defined by a diameter in the boundary portion between the second core portion 120 and the third core portion 130 at which the relative refractive index difference is (xcex94n2+xcex94n3)/2; and the outer diameter c of the second core portion 130 is defined by a diameter in the boundary portion between the third core portion 130 and the cladding portion 200 at which the relative refractive index difference is (xcex94n3+xcex94nc)/2. Here, in the specification, since the mean relative refractive index differences of the respective glass regions 110, 120, and 130 are defined on the basis of the refractive index of the cladding portion 200, the mean relative refractive index difference xcex94nc of the cladding portion 200 with respect to itself is 0.
Normally, in the SM optical fiber, two phenomenons, namely, a material dispersion in which velocity of propagation toward the longitudinal direction (direction in which signal light advances) becomes faster as the signal light has a longer wavelength and a structure dispersion in which velocity of propagation becomes faster as the signal light has a shorter wavelength, occur concurrently. Accordingly, such an SM optical fiber has a refractive index profile in which the refractive index near the core center has a constant value or decreases toward the outside. Here, gradients of the material dispersion and structure dispersion with respect to wavelength have polarities opposite to each other, while the material dispersion normally has a larger absolute value of gradient. Accordingly, in such an SM optical fiber, the total dispersion obtained as the material dispersion and structure dispersion are added together has a unique gradient (dispersion slope) inherent in each SM optical fiber with respect to the zero dispersion wavelength.
The inventors have found out that, in the core portion of SM optical fibers having a refractive index profile of dual shape type or segmented-core type, when the relative refractive index difference of the center part of the core with respect to the cladding portion is reduced as compared with that of the core at the peripheral part, the absolute value of dispersion slope can be reduced without decreasing MFD or increasing the bending loss. Namely, in the refractive index profile, according as the region (indent indicated by mark xe2x80x9cAxe2x80x9d in the refractive index profile of FIG. 1) corresponding to the center part of the core is wider and deeper (a difference between the first and second values xcex94n1 and xcex94n2 is larger), the absolute value of dispersion slope can be reducer. In particular, the inventors have confirmed that the width of this indent A has a great effect on reducing the absolute value of dispersion slope, such that, when the width of this indent is insufficient (narrow), the effect on reducing the absolute value of dispersion slope can hardly be obtained. In this specification, xe2x80x9cdispersion slopexe2x80x9d used alone indicates its absolute value.
As disclosed in xe2x80x9cLow-Loss Dispersion-Shifted Single-Mode Fiber Manufactured by the OVD Process,xe2x80x9d Journal of Lightwave Technology, Vol. LT-3, No. 5, p. 931, 1985, it has been known that a deep crack is formed at the region corresponding to the center part of the core portion in the refractive index profile of an SM optical fiber manufactured by OVD (Outside Vapor Deposition) method. This crack, however, is not intended for reducing the dispersion slope (absolute value of dispersion slope). Rather, it is non-intentionally generated as a matter of convenience in manufacture, and the aimed effects of the present invention cannot be expected since the width of this crack is too narrow. Also, though a crack such as that mentioned above is generated in MCVD (Modified Chemical Vapor Deposition) method, no effect on reducing the dispersion slope can be expected for the same reasons as those of the above-mentioned OVD method.
In the SM optical fiber 1 according to the present invention, by contrast, since the indent A in its refractive index profile 600 has a sufficient width (since the first core portion 110 and the second core portions 120 satisfy the above-mentioned relationship (1)), the absolute value of gradient of structure dispersion with respect to wavelength becomes smaller, thereby enabling the reducing of dispersion slope in a wide wavelength range.
Here, in order to attain a greater effect on reducing the dispersion slope as compared with the conventional SM optical fiber, it is preferable that the third value xcex94n3 be set to 0.03% or higher, and the second value xcex94n2 be set to 0.4% or higher.
Also, the inventors have confirmed that, according to the requirement (that bending loss at a diameter of 32 mm (referred to as xe2x80x9c32 mmxcfx86 bending lossxe2x80x9d hereinafter) be 0.50 dB/turn or less with respect to light having a-wavelength of 1,550 nm) indicated in Standard 4.2.6 Fiber Macrobend (Generic Requirement for Optical Fiber and Fiber Optic Cable, GR-20-CORE, Issue Sep. 1, 1994) published by Bellcore of USA, b/c xe2x89xa60.4. Here, since the cut-off wavelength becomes longer than the wavelength of the intended signal light when b/cxe2x89xa60.1, it is preferable that 0.1xe2x89xa6b/cxe2x89xa60.4. Further, the second core-portion 120 and the third core portion 130 preferably satisfy the relationship of 0.1xe2x89xa6b/cxe2x89xa60.3 therebetween. It is due to the fact that the bending loss at a diameter of 30 mm (30 mmxcfx86 bending loss) of 0.1 dB/turn or less is generally recognized as a standard for preventing the transmission loss from increasing in the SM optical fiber applied to a cable of a tight configuration.
Also, the inventors have confirmed that, as the value of the above-mentioned relational expression (1) is increased, the effect on reducing the dispersion slope is maximized at about 0.5, whereas the dispersion slope rather increases thereafter (see FIG. 15). Accordingly, the first core portion 110 and the second core portion 120 preferably satisfy the following relationship:
axc2x7(xcex94n2xe2x88x92xcex94n1)/(bxc2x7xcex94n2)xe2x89xa60.5xe2x80x83xe2x80x83(2)
therebetween.
Further, in the SM optical fiber 1 according to the present invention, as shown in FIG. 3, the third core portion 130 is preferably constituted by an inner core 130a which is a glass region whose mean relative refractive index difference with respect to the cladding portion 200 is a fourth value xcex94n3a ( less than xcex94n2) and whose outer diameter is c4 ( less than c) formed around the outer periphery of the second core portion 120 and an outer core 130b which is a glass region whose mean relative refractive index difference with respect to the cladding portion 200 is a fifth value xcex94n3b ( less than xcex94n2 and  greater than xcex94n3a) and whose outer diameter is c spaced from the second core portion 120 by way of the inner core 130a. In other words, in the refractive index profile 700 of FIG. 3, an indent B is formed at a region corresponding to the third core portion 130 (including the inner core 130a and the outer core 130b). Here, the inner core 130a and the outer core 130b satisfy the following relationship:
0.1xe2x89xa6(c4xe2x88x92b)xc2x7(xcex94n3bxe2x88x92xcex94n3a)/(cxc2x7xcex94n3b)xe2x89xa60.8xe2x80x83xe2x80x83(3)
therebetween.
When the indent B having sufficient width and depth is provided in the profile region corresponding to the third core portion 130, as in the case of the effect obtained by the above-mentioned indent A, seepage of light into the cladding portion 200 can be made greater, thereby decreasing the gradient (absolute value) of structure dispersion with respect to wavelength and consequently reducing the dispersion slope.
An effect on reducing the dispersion slope is obtained at the lower limit of the above relational expression (3) or higher, while being maximized at the upper limit thereof. Beyond this upper limit, the dispersion slope rather increases (see FIG. 16).
The first method of manufacturing the SM optical fiber according to the present invention comprises, as shown in FIG. 5, a first step of preparing a glass tube 201 which is to be the cladding portion 200 having a predetermined refractive index, and flowing a material gas containing at least Si and Ge through a hollow part of the glass tube 201, while heating the glass tube 201, thereby forming, on the inner surface of the glass tube 201, a first soot body 131 which is to be the third core portion 130 after vitrification, the third core portion having a mean relative refractive index difference with respect to the cladding portion 200 of the third value xcex94n3; a second step of flowing a material gas containing at least Si and Ge through the hollow part of the glass tube 201 in which the first soot body 131 is formed, while heating the glass tube 201, thereby forming, on the inner surface of the first soot body 131, a second soot body 151 which is to be the first core portion 110 and the second core portion 120 after vitrification, the second core portion having a mean relative refractive index difference with respect to the cladding portion 200 of the second value xcex94n2 ( greater than xcex94n3), the first core portion 110 having a mean relative refractive index difference with respect to the cladding portion 200 of the first value xcex94n1 ( less than xcex94n2); a third step of flowing a halogen gas through the hollow part of the glass tube 201 in which the first soot body 131 and the second soot body 151 are formed, while heating the glass tube 201, thereby diffusing germanium contained in the inner surface side of the second soot body 151 so as to form an inner region 111 and an outer region 121 respectively having the germanium concentrations different from each other; a fourth step of heating and collapsing the glass tube 210 in which the first soot body 131 and the inner and outer regions 111 and 121 of the second soot body 151 are formed, thereby obtaining a transparent optical fiber preform 310; and a fifth step of drawing one end of thus obtained optical fiber preform 310 while heating it, thereby yielding the SM optical fiber 1 having the refractive index profile 600 shown in FIGS. 1 and 2A-2C.
This first manufacturing method belongs to the MCVD method. In particular, in the third step, a halogen gas such as chlorine is flowed through the hollow part of the glass tube 201, while the latter is heated, so as to diffuse germanium on the inner surface side of the second soot body 151, thereby intentionally reducing the relative refractive index difference of the inner region 111 to be the first core portion 110 after vitrification with respect to that of the outer region 121 to be the second core portion 120 after vitrification. Accordingly, thus obtained SM optical fiber 1 has a refractive index profile 600 in which the indent A with a sufficient width is formed at the center part of the core region.
Here, in order to obtain the SM optical fiber 1 having the refractive index profile 700 shown in FIGS. 3 and 4A-4C, the above-mentioned third step is effected at the above-mentioned first step.
The second method of manufacturing the SM optical fiber according to the present invention comprises, as shown in FIG. 7, a first step of forming a first soot body 112 to be the first core portion 110 after vitrification, the first core portion 110 having a mean relative refractive index difference with respect to the cladding portion 200 of the first value xcex94n1; a second step of forming, around the outer periphery of the first soot body 112, a second soot body 122 to be the second core portion 120 after vitrification, the second core portion 120 having a mean relative refractive index difference with respect to the cladding portion 200 of the second value xcex94n2 ( greater than xcex94n1); a third step of forming, around the outer periphery of the second soot body 122, a third soot body 132 to be the third core portion 130 after vitrification, the third core portion 130 having a mean relative refractive index difference with respect to the cladding portion 200 of the third value xcex94n3 ( less than xcex94n2); a fourth step of forming, around the outer periphery of the third soot body 132, a fourth soot body 212 to be the cladding portion 200 having a predetermined refractive index after vitrification; a fifth step of heating and collapsing a composite soot body 321 formed at the fourth step, thereby obtaining a transparent optical fiber preform 320; and a sixth step of drawing one end of thus obtained optical fiber preform 320 while heating it, thereby yielding the SM optical fiber 1 having the refractive index profile 600 shown in FIGS. 1 and 2A-2C.
This second manufacturing method belongs to the VAD method. In particular, in the first step, a portion which is to be the first core portion 110 after vitrification and is for forming the indent A with a sufficient width in the refractive index profile 60D is intentionally formed. Accordingly, thus obtained SM optical fiber 1 has a refractive index profile 600 in which the indent A with a sufficient width is formed at the center portion of the core region.
Here, in order to obtain the SM optical fiber 1 having the refractive index profile 700 shown in FIGS. 3 and 4A-4C, the third step is constituted by a first sub-process of forming, around the outer periphery of the second soot body 122, an inner soot body to be the inner core 130a of the third core portion 130 after vitrification, the inner core 130a being formed around the outer periphery of the second core portion 120 and having a mean relative refractive index difference with respect to the cladding portion 200 of the fourth value xcex94n3a ( less than xcex94n2); and a second sub-process of forming, around the outer periphery of the inner soot body to be an outer core 130b of the third core portion 130 after vitrification, the outer core 130b being formed around the outer periphery of the inner core 130a and having a mean relative refractive index difference with respect to the cladding portion 200 of the fifth value xcex94n3b ( less than xcex94n2 and  greater than xcex94n3a).
The third method of manufacturing the SM optical fiber according to the present invention comprises, as shown in FIG. 9, a first step of forming, around the outer periphery of a cylindrical glass rod 500, a first soot body 153 to be the first core portion 110 and the second core portion 120 after vitrification, the first core portion 110 having a mean relative refractive index difference with respect to the cladding portion 200 of the first value xcex94n1, the second core portion 120 having a mean relative refractive index difference with respect to the cladding portion 200 of the second value xcex94n2 ( greater than xcex94n1); a second step of forming, around the outer periphery of the first soot body 153, a second soot body 133 to be the third core portion 130 after vitrification, the third core portion having a mean relative refractive index difference with respect to the cladding portion 200 of the third value xcex94n3 ( less than xcex94n2); a third step of forming, around the outer periphery of the second soot body 133, a third soot body 213 to be the cladding portion 200 having a predetermined refractive index after vitrification; a fourth step of pulling out the glass rod 500 and flowing a halogen gas through a hollow part of a tubular soot body 331 comprising the first soot body 153, second soot body 133, and third soot body 213, while heating the tubular soot body 331, thereby diffusing germanium contained in the inner surface side of the first soot body 153 so as to obtain an inner soot body 113 and an outer soot body 123 respectively having the germanium concentrations different from each other; and a fifth step of heating and collapsing this tubular soot body 331 so as to obtain a transparent optical fiber preform 330; and a sixth step of drawing one end of thus obtained optical fiber preform 330 while heating it, thereby yielding the SM optical fiber 1 having the refractive index profile 600 shown in FIGS. 1 and 2A-2C.
This third manufacturing method belongs to the OVD method. In particular, in the fourth step, a halogen gas such as chlorine is flowed through the hollow part of the tubular soot body 331, while the latter is heated, so as to diffuse germanium on the inner surface side of the first soot body 153, thereby intentionally reducing the germanium concentration of the inner soot body 113 as compared with that of the outer soot body 123. Accordingly, thus obtained SM optical fiber 1 has a refractive index profile 600 in which the indent A with a sufficient width is formed at the center portion of the core region.
Here, in order to obtain the SM optical fiber 1 having the refractive index profile 700 shown in FIGS. 3 and 4A-4C, the second step is constituted by a first sub-process of forming, around the outer periphery of the first soot body 153, an inner soot body to be the inner core 130a of the third core portion 130 after vitrification, the inner core 130a being formed around the outer periphery of the second core portion 120 and having a mean relative refractive index difference with respect to the cladding portion 200 of the fourth value xcex94n3a ( less than xcex94n2); and a second subprocess of forming, around the outer periphery of the inner soot body, an outer soot body to be an outer core 130b of the third core portion 130 after vitrification, the outer core 130b being formed around the outer periphery of the inner core 130a and having a mean relative refractive index difference with respect to the cladding portion 200 of the fifth value xcex94n3b ( less than xcex94n2 and  greater than xcex94n3a).
The fourth method of manufacturing the SM optical fiber according to the present invention comprises, as shown in FIG. 11, a first step of forming, around the outer periphery of the cylindrical glass rod 500, a first soot body 124 to be the second core portion 120 after vitrification, the second core portion 120 having a mean relative refractive index difference with respect to the cladding portion 200 of the second value xcex94n2; a second step of forming, around the outer periphery of the first soot body 124, a second soot body 134 to be the third core portion 130 after vitrification, the third core portion 130 having a mean relative refractive index difference with respect to the cladding portion 200 of the third value xcex94n3 ( greater than xcex94n2); a third step of forming, around the second soot body 134, a third soot body 214 to be the cladding portion 200 having a predetermined refractive index after vitrification; a fourth step of pulling out the glass rod 500, and heating and sintering the first soot body 124, the second soot body 134, and the third soot body 214; a fifth step of inserting, into a hollow part of a sintered body 341 formed at the fourth step, a cylindrical glass rod 114 to be the first core portion 110 after vitrification, the first core portion 110 having a mean relative refractive index difference with respect to the cladding portion 200 of the first value xcex94n1 ( less than xcex94n2), and heating and integrating thus formed composite body so as to obtain a transparent optical fiber preform 342; and a sixth step of drawing one end of a finally obtained optical fiber preform 340 while heating it, thereby yielding the SM optical fiber 1 having the refractive index profile 600 shown in FIGS. 1 and 2A-2C.
This fourth manufacturing method belongs to the OVD method. In particular, in the fourth step, the glass rod 114 to be the first core portion 110 after vitrification is inserted into the sintered body 341, and thus formed composite body is integrated so as to obtain the optical fiber preform 340. Accordingly, thus obtained SM optical fiber 1 has a refractive index profile 600 in which the indent A with a sufficient width is formed at the center portion of the core region.
Here, in order to obtain the SM optical fiber 1 having the refractive index profile 700 shown in FIG. 3 and 4A-4C, the second step is constituted by a first sub-process of forming, around the outer periphery of the first soot body 124, an inner soot body to be an inner core 130a of the third core portion 130 after vitrification, the inner core 130a being formed around the outer periphery of the second core portion 120 and having a mean relative refractive index difference with respect to the cladding portion 200 of the fourth value xcex94n3a ( less than xcex94n2); and a second sub-process of forming, around the outer periphery of the inner soot body, an outer soot body to be an outer core 130b of the third core portion 130 after vitrification, the outer core 130b being formed around the outer periphery of the inner core 130a and having a mean relative refractive index difference with respect to the cladding portion 200 of the fourth value xcex94n3b ( less than xcex94n2 and  greater than xcex94n3a).
An SM optical fiber 10 according to the second embodiment of the present invention comprises, as shown in FIG. 22, an inner core portion 150 having a mean relative refractive index difference with respect to an outer cladding portion 260 of xcex94n4 and an outer diameter of d; an outer core portion 160 formed around the outer periphery of the inner core portion 150, the outer core portion 160 having a mean relative refractive index difference with respect to the outer cladding portion 260 of xcex94n5 ( less than xcex94n4) and an outer diameter of e; an inner cladding portion 250 formed around the outer periphery of the outer core portion 160, the inner cladding portion 250 having a mean relative refractive index difference with respect to the outer cladding portion 260 of xcex94n6 ( less than xcex94n5 and  less than 0) and an outer diameter of f; and the outer cladding portion 260 formed around the outer periphery of the inner cladding portion 250, the outer cladding portion 260 having a predetermined refractive index (higher than that of the inner cladding portion 250). Namely, as shown in FIG. 22, this SM optical fiber 10 has a refractive index profile 800 with an indent C.
In particular, the inner cladding portion 250 and the outer cladding portion 260 satisfy the following relationship:
exc2x7|xcex94n4|/(fxe2x88x92e)xe2x89xa70.03
therebetween.
Here, the refractive index profile 800 of the SM optical fiber 10 does not always have a clear step-like form. In such a case, the outer diameter d of the inner core portion 150 is defined by a diameter in the boundary portion between the inner core portion 15Q and the outer core portion 160 at which the relative refractive index difference is (xcex94n4+xcex94n5)/2; the outer diameter e of the outer core portion 160 is defined by a diameter in the boundary portion between the outer core portion 160 and the inner cladding portion 250 at which the relative refractive index difference is (xcex94n, +xcex94n4)/2; and the outer diameter f of the inner cladding portion 250 is defined by a diameter in the boundary portion between the inner cladding portion 250 and the outer cladding portion 260 at which the relative refractive index difference is (xcex94n6+xcex94nc)/2. Here, in the specification, the mean the relative refractive index differences of the respective glass regions 150, 160, and 250 are defined on the basis of the refractive index of the outer cladding portion 260, the means relative refractive index difference xcex94nc of the outer cladding portion 260 with respect to itself is 0.
The inventors have also confirmed that, when a region having a low refractive index (inner cladding portion 250) is disposed outside of the core region as in the case of the above-mentioned configuration, the dispersion slope can be reduced without any decrease in MFD. The effect on reducing the dispersion slope becomes greater as the indent C in the refractive index profile 800 is wider or deeper. Since this profile leads to increase in bending loss, however, there is a limit to the reducing of dispersion slope. As an SM optical fiber of a type in which relative refractive index difference of the glass region around the core is lowered, a fiber having a refractive index profile disclosed in Japanese Patent Laid-Open No. Sho 63-43107 has been suggested. In this publication, one of objects is to decrease glass additives, while attaining a higher relative refractive index difference at the core region (without changing the form of profile).
Nevertheless, Japanese Patent Laid-Open No. Sho 63-43107 does not mention the width and depth of the inner cladding. Though its specification states a width within the range of 1 to 35 xcexcm or 1b to 10b (wherein b is inner core diameter; b=1 to 10 xcexcm) and a depth within the relative refractive index difference range of xe2x88x920.1% to xe2x88x920.6%, bending loss drastically increases in most part of these ranges.
By contrast, as in the case of the SM optical fiber 10, when the inner cladding portion 250 whose mean relative refractive index difference with respect to the outer cladding portion 260 is xcex94n6 ( less than 0) is formed inside of the outer cladding portion 260, seepage of signal light from the inner cladding portion 250 to the outer cladding portion 260 becomes greater on the long wavelength side in particular. Accordingly, the gradient (absolute value) of structure dispersion with respect to wavelength becomes smaller, thereby reducing the gradient (dispersion slope) of the total dispersion which includes the material dispersion in addition to the structure dispersion.
Here, in the case where f/e greater than 4, even when the relative refractive index difference xcex94n6 ( less than 0) of the inner cladding portion 250 is made smaller so as to constantly hold the value of exc2x7|xcex94n4|/(fxe2x88x92e), the effect on reducing the dispersion slope can hardly be obtained. Accordingly, it is preferable that f/exe2x89xa64.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.